The present invention relates to the use of powders in the form of a ceramic shell that may optionally be filled or void in the production of thermally sprayed coatings on substrates.
It is well known that placing a temperature-resistant ceramic coating on a substrate can enable the substrate to withstand higher temperatures without deterioration and this has become a common way of protecting substrates as diverse as molds used for casting metals and in the hot zones of turbine engines such as for example on turbine blades for jet engines. The coatings can be applied by a number of techniques but the most frequently used is based on the use of a thermal spray in which the ceramic particles are injected into a plasma jet directed towards the substrate. The heat of the plasma jet melts the ceramic particles and causes them, upon impact with the substrate, to form a ceramic layer with a high degree of uniformity and integrity that is capable of protecting the substrate on which it is coated, giving the substrate the superficial thermal and wear characteristics of the ceramic with which it is coated.
There is however a problem when it is desired to use a thickness of coating that is more than about 250 micrometers in thickness since there is a tendency for the coating to spall off the substrate, especially if there is extensive thermal cycling or if the coated substrate is subjected to impacts. Spalling can also occur during spraying of the powder. This is believed to be due at least in part to a certain lack of conformity of the sprayed material with the surface of the substrate. One way to handle this problem is to use a thermal spray in which polymer particles are injected into the cooler part of a plasma flame while ceramic particles are injected into the hotter part leading to co-deposition of polymer and ceramic particles. This process, which is described in U.S. Pat. No. 6,051,279, gives the layer a degree of ability to absorb strains and to survive conditions that would lead to spalling of a purely ceramic layer. It is thus possible to deposit much greater thicknesses of protective coating. As might be imagined however, there is great difficulty in controlling the temperature of the flame and the rates of injection so as to get a uniform coating. It is also known to plasma spray hollow ceramic oxide particles though the method by which such particles are made does not preserve the hollow structure to any significant degree and these also suffer from uniformity problems and spalling when coatings with significant thicknesses, (over about 250 micrometers), are attempted.
The problem is magnified further when the ceramic to be deposited is zirconia. As is well known in the art, the tetragonal form of zirconia is the high temperature stable form and that, as the temperature declines below the transition temperature, the zirconia changes the crystalline structure to the monoclinic form, which is the low temperature-stable form, and undergo dimensional and volume changes. Thus under plasma spray conditions the zirconia will be in the tetragonal form but will on cooling revert to the monoclinic form and this leads to a source of imperfections in the coating. Indeed such coatings are quite unable to stand any significant thermal cycling to a point above the transition temperature to the tetragonal form. It is however known that some metal oxides such as magnesia, yttria and certain transition metal oxides have the capability of stabilizing the zirconia in the tetragonal form during such thermal cycling and it is known to add such oxides in admixture with zirconia to the plasma flame. This however produces inhomogeneities of the zirconia structure as a result of incomplete stabilization and this too has its problems.
There is therefore a need for a process for the production of thermally sprayable ceramic powders that can be deposited to form protective layers that are relatively thick (such as 250 micrometers or more) on a substrate and the present invention provides just such a process.
The present invention provides a process for coating a substrate which comprises:
a) forming a suspension comprising ceramic particles and particles of a combustible material in which the average particle sizes of the combustible and ceramic components are in a ratio of from 100:1 to 5:1 and the combustible particles have an average particle size of from 10 to 500 micrometers;
b) granulating the suspension, for example by spray drying, to form composite particles in which the particles of combustible material are coated with the particles of the ceramic; and
c) plasma spraying the composite particles on to the substrate.
The composite particles obtained can be used to coat a substrate allowing a much greater thickness of coating to be obtained and, if the nature of the combustible material is chosen appropriately, a coating with much greater conformability to the substrate and therefore a much reduced danger of a coating subject to spalling.
Alternatively the composite particles can be heated in a furnace at a temperature high enough to burn out the combustible material leaving stable hollow ceramic particles. These can then be used directly to build up plasma sprayed protective coatings in which the particles are fed directly into a plasma flame at a temperature sufficient to melt the ceramic and are deposited as a layer on a substrate.
The ceramic layer can be any of the ceramic oxides used for such purposes including but not limited to zirconia, yttria-stabilized zirconia, alumina, chromia, magnesia and mixtures of such oxides including spinel structures.
The combustible material is preferably a polymer that is stable at temperatures of at least 100xc2x0 C., and more preferably above about 300xc2x0 C., such as for example polyesters, nylon, polyimides and polycarbonates. It can also be for example a readily combustible finely divided organic material powder such as walnut shell flour or wood powder.
The ceramic-shelled particles for use in a preferred process according to the invention are made by mixing particles of a combustible polymer component and a ceramic oxide component in the form of stable aqueous dispersions and forming the dispersed mixture into composite particles in which the ceramic particles are coated upon the polymer particles, for example by spray drying or by one of the spray granulation processes known in the art. Stability of the dispersion is preferably enhanced by the use of a surfactant and the adhesion of the ceramic particles to the polymer particles is most suitably facilitated by the use of a temporary binder. The ceramic oxide particle size is smaller than that of the polymer particles such that the ceramic particles form a uniform coat around the polymer particles. Typical polymer average particle sizes in polymer dispersions are from 10 to about 500 micrometers, such as from 50 to 400 micrometers, and most preferably from 100 to 300 micrometers. The ceramic oxide particles have average particle sizes that are from 0.5 to 100 micrometers, such as from 1 to 50 and preferably from 5 to 20 micrometers. In the context of this Application, xe2x80x9caverage particle sizexe2x80x9d shall mean the volume average particle size as determined for example using a xe2x80x9cMICROTRACxe2x80x9d laser light scattering device. In a preferred embodiment of the invention the volume proportions of ceramic and polymer are from 1:100 to 1:5 and preferably from 1:20 to 1:10.